So far I've used these 4 submission services:
- shortfilmdepot (reliable)
- reelport (and reelport beta, which is a design nightmare) (reliable)
- festhome (just because of this one festival)
- filmfreeway (spams me with all those submission fee festivals, has nearly no free submissions)

Many festivals still rely on their own submission process, with more or less userfriendly solutions. Some require to create an account first.

With all those services, accounts, and stuff, it's a very good idea to make a list of all this stuff, otherwise you'll be pretty easily overwhelmed and will miss deadlines (happened to me). I made a spreadsheet with fields for:
- festival
- town
- URL
- mail (some festivals contact you via mail, try to find this mail again after a month!)
- date of festival
- submission deadline
- date of confirmation, if given
- required screener format: some still want DVDs
- Vimeo link, or service used, or their own service (account name & PW)
- accepted or rejected (I made this bright red/green)
- accepted projection file format (some want DCP & mov/mp4 as backup, some wanted ProRes!)
- language version / subtitles
- post / download / FTP upload
- projection file deadline
- projection file sent
- if USB stick: received after festival (Annecy seems to keep ...)
You'll start with many empty fields, get used to fill in missing information as soon as you'll get it.

You'll need to organize your mail. I have one folder for each festival which accepts my film, one for invitations, one for rejections. A single festival may have a lot of different email addresses.

There's a mostly useful list here: http://www.animation-festivals.com, I recommend to browse festivals and their terms first, then decide which services to use, as only very few festivals use more than one.

Things you should've prepared before submitting or using a submission service:
- a Vimeo link with PW for screening (allow embedding)
- a DCP file - on USB stick or HD (NTFS formatted), and as tested .ZIP file (festival Mac users may encounter problems, tell them to try 7yx or StuffitExpander instead of their normal utility)
- a .mov and an .mp4 file, codec h.264 is widely accepted, some ask for Apple ProRes
- a link to some file transfer service - I use wetransfer.com, free up to 2 GB, and mega.nz, free up to 50 GB. The latter offers an encrypted link, so you can store your DCP there, if you dare. There are other services as well.
- 3 images from your film, 1920 x 1080px, as .jpg (set to 300 dpi before, some festivals are quite picky without enough print knowledge)
- 1 image of yourself
- synopsis, one long, one short (.txt file) (- in several languages)
- dialogue list, better .srt file for subtitles
- list of team members and their roles
- director's bio and film list (.txt file) (- in several languages)
- press kit (can be one .pdf, or several files in a .zip)
- addresses of yourself, your producer, your distributor

Some festivals (especially in Spain, don't know why) ask for a signed agreement, usually that's a PDF, be prepared to have a recent Acrobat Reader installed, and a digital signature. Alternatively do a screenshot of their PDF and sign it in TVP/Photoshop, and send as .jpg.

Language versions and subtitles: read the rules carefully since many festivals are quite picky. Some (like Annecy) offer to do translation and subtitles themselves. Most want subtitles "baked" into the film, only few accept .srt files. Favourite languages:
- english (nearly everybody accepts that)
- french (french festivals demand this often)
- spanish (Spain, and whole South America - don't miss this, there's a lot of festivals there)

Fees: So far I've avoided all festivals which ask for a submission fee, but if you have a budget for this, you don't have to do the same. It's only even more difficult to weed out the good festivals from the not-so-good ones. Have a look at the awards they announce to give, and the relation between film screenings and "business/industry opportunities" like panels, talks, workshops - you can only profit from those if you're there, and that means travel expenses. A combination of fees and no competition is a bad sign.

Some festivals will send showcase programs to other festivals, in this case the latter should offer a fee.